The invention concerns a method for nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) measurements, comprising the steps:                (i) creation of enhanced polarisation of nuclei of a first kind within a sample in a magnetic field at cryogenic temperatures; and        (ii) transfer of the polarised sample to room temperature.        
A method as described above is known from reference 3.
The limited sensitivity of magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) experiments may be considerably improved by coupling the nuclear spins to energy states affording a larger polarisation such as those of electron spins, whereby “polarisation” is used as synonym for “longitudinal magnetisation”. Nuclear hyperpolarisation using the nucleus-electron spin-spin coupling may be achieved in two ways: (i) in-situ DNP experiments for polarisation enhancement in the magnet where signal detection is performed. These experiments can be performed repetitively at short time intervals in solid samples using gyrotrons1 or in liquid samples2 by relying on the Overhauser effect; (ii) ex-situ experiments, such as dissolution DNP3, where the polarisation of electron spins is transferred to nuclei in a polarising magnet prior to NMR one-shot detection in a high-resolution magnet. This approach can yield enhancements of up to four orders of magnitude, but since it takes a long time to build up the enhanced polarisation at temperatures of ca. 1.2 K, one can typically only perform a few experiments per day. The intrinsically short lifetimes of hyperpolarised polarisation, normally determined by the longitudinal relaxation time constants T1, compel one to make use of the enhanced polarisation quickly, within intervals of the order of a few tens of seconds. Therefore, reactions or transport phenomena that occur on longer time scales cannot be followed using dissolution DNP.
The longitudinal relaxation times T1 of the populations of ordinary Zeeman states of spins I=½ are usually limited by dipole-dipole couplings and anisotropic chemical shifts, both modulated by molecular tumbling. The dipolar interaction is often the dominant source of relaxation. It has been shown by Levitt and co-workers4, 6 that this mechanism may be silenced under particular circumstances, by rendering the spins equivalent, either through the application of a suitable radio frequency (rf) irradiation, or by the removing the sample from the magnetic field. Spin order, under these conditions, may be preserved as long-lived states (LLS), which, in the simple case of a system consisting of two coupled spins ½, correspond to a difference between the populations, on the one hand, of the singlet state
                    S        0            =                        1                      2                          ⁢                  (                                                  αβ              〉                        -                                    ⁢        βα              〉    )and, on the other hand, of the mean population of the three triplet states
                                                                                          T                                      +                    1                                                  =                                  |                  αα                                            〉                        ,                                          T                0                            =                                                1                                      2                                                  ⁢                                  (                                                                                  αβ                      〉                                        +                                                                    ⁢                βα                                              〉                )            ,                          ⁢                        and          ⁢                                          ⁢                      T                          -              1                                      =                  |          ββ                      〉    .
This difference can be represented by the operator:QLLS=−N{right arrow over (I)}·{right arrow over (S)}=−N(IxSx+IySy+IzSz)=−N(IzSz+ZQx)  (1)with the norm
  N  =      -                  2                  3                    .      
It has been shown5,7-12 that long-lived states, i.e., states with lifetimes that are partly immune to dipolar relaxation, can exist in systems which may comprise as many as five coupled spins. These states may be associated with sensitive proton spins in endogenous substances such as glycerol or taurine and a variety of amino acids (Gly, Asp, Asn, Cys), particularly when these belong to mobile parts of proteins. A six-fold increase in the relaxation time constant (TLLS/T1=6) has been determined for glycine residues in the C-terminus of Ubiquitin13.
Applications have been developed to exploit long-lived spin states for the study of slow diffusion13-16, exchange17, and for sustaining enhanced magnetisation10, 11, 18.
The latter application has been intensely pursued because of the high costs, both in terms of time and equipment, which are involved in obtaining nuclear hyperpolarisation. Provided the population differences can be enhanced in organic molecules, and provided this polarisation can be sustained during in vivo studies, it may be possible to use endogenous substances to follow metabolic pathways. Increasing the magnetisation lifetime in polarised molecules will also lead to better spatial and temporal resolution in magnetic resonance imaging19. Some systems can sustain LLS transferred from the polarisation of para-hydrogen10, 11. In particular, in molecules featuring chemically equivalent nuclei, spin states with relatively long lifetimes are believed to exist. Their enhanced magnetisation may be converted to detectable magnetisation using hydration reactions18.
Object of the invention is to present a method for nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) measurements that allows one to extend the time needed between hyperpolarised magnetisation and NMR detection.